1. Field of the Invention
Aspects of the present invention relate to a switch mode power supply apparatus, and more particularly, to a switch mode power supply apparatus having an active clamping circuit which can suppress a voltage stress of a switching device to adjust a power supply supplied to a primary coil of a transformer of the switch mode power supply apparatus.
2. Description of the Related Art
In conventional switch mode power supply apparatuses, particularly, in flyback converters, forward converters, etc., when a switch operation is performed, a voltage spike is generated across a main switching device due to energy which is stored in a leakage inductor or magnetizing inductor of a transformer. Due to the voltage spike, power loss increases and an excessive voltage stress is applied to the main switching device. In order to suppress the voltage stress, an active clamping circuit to form a discharge path for the energy stored in the leakage inductor or magnetizing inductor is used. The active clamping circuit operates when the main switching device opens so as to prevent the main switching device from being damaged due to the energy stored in the leakage inductor or magnetizing inductor and reuse the energy, thereby improving power conversion efficiency. Also, by using the active clamping circuit, zero-voltage switching between the main switching device and a clamping switching device of the active clamping circuit is possible. Accordingly, switching loss is reduced, thus increasing efficiency, and the amount of heating upon switching is reduced.
However, when an active clamping circuit is used in a standby mode in which power consumption of the switch mode power supply apparatus becomes very low, power loss increases due to copper loss or load loss of a transformer of the switch mode power supply apparatus. The copper loss is caused by a circulating current of a primary side of the transformer, which is generated by a clamping operation, and the reduction of power consumption of the switch mode power supply apparatus in the standby mode is limited. Also, due to the clamping operation of the active clamping circuit, since the switch mode power supply apparatus operates only in a continuous conduction mode (CCM), it is difficult to enter a burst mode or a skip cycle mode in which switching loss can be reduced, and accordingly power loss further increases in the standby mode.